Techniques to manage dwell times for pilot rotation

ABSTRACT

Techniques to manage dwell times for pilot rotation are described. An apparatus may comprise a memory configured to store a data structure with a set of modulation and coding schemes (MCS) available to an orthogonal frequency division multiplexing (OFDM) system, each MCS having an associated pilot dwell time. The apparatus may further comprise a processor circuit coupled to the memory, the processor circuit configured to identify a MCS to communicate a packet using multiple subcarriers of the OFDM system, and retrieve a pilot dwell time associated with the MCS from the memory, the pilot dwell time to indicate when to shift a pilot tone between subcarriers of the multiple subcarriers during communication of the packet. Other embodiments are described and claimed.

RELATED APPLICATION

This application is a Continuation-In-Part (CIP) of, and claims priorityto, commonly owned U.S. patent application Ser. No. 13/628,613 entitled“IMPROVED CHANNEL ESTIMATION AND TRACKING” (Docket No. P43789) filedSep. 27, 2012, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND

Sensor networks have numerous applications, such as security, industrialmonitoring, military reconnaissance, and biomedical monitoring. In manysuch applications, it is either inconvenient or impossible to connectthe sensors by wire or cable; a wireless network is preferable. Sensornetworks may be implemented indoors or outdoors. Seismic sensors, forexample, may be used to detect intrusion or movement of vehicles,personnel, or large earth masses.

The detection of vehicles and personnel is more difficult than detectinglarge signals, as from earthquakes or movement of earth masses. Thereliable detection or tracking over large areas thus requires very largenumbers of sensitive detectors, spaced closely. Although placing sensornodes in the environment is relatively easy, and configuring them in anetwork is manageable, a problem faced by sensor networks is thatdetermining where they are in geographic coordinate locations isdifficult and expensive. A wireless network of numerous sensitive, lowcost, low-powered sensor stations is more desirable. However, theresulting overhead for channel estimation is usually prohibitive in awireless sensor network.

A wireless communications standard is being developed by the Instituteof Electrical and Electronics Engineers (IEEE) 802.11ah (11ah) taskgroup. IEEE 802.11ah (11ah) is a new technology evolution for WiFi andis in the standards development phase; very low data rate operation isbeing enabled. In IEEE 802.11a/g, 20 MHz channel widths were defined andin IEEE 802.11n 40 MHz was added and then in IEEE 802.11ac both 80 and160 MHz. In the past the evolution of WiFi has been to increase datarate, but IEEE 802.11ah (11ah) actually targets comparatively lower rateservices.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1A illustrates the concept of sensor network deployment inaccordance with an embodiment;

FIG. 1B is an exemplary communication device suitable for implementingdifferent embodiments of the disclosure;

FIG. 1C is a diagram with fixed pilots in accordance with oneembodiment;

FIG. 2 is a first diagram of a packet/frame with pilot tones transmittedby a transmitter in accordance with an embodiment;

FIG. 3 illustrates a method for pilot shifting in an orthogonalfrequency division multiplexing (OFDM) based communication system inaccordance to an embodiment;

FIG. 4 is an illustrates part of a transceiver with equalizer forprocessing pilot tones and data tones in accordance with an embodiment;

FIG. 5 is a flowchart of a method for tone allocation in a transmitterin accordance with an embodiment;

FIG. 6 is an exemplary pilot dwell time table in accordance with oneembodiment;

FIG. 7 is a second diagram of a packet/frame with pilot tonestransmitted by a transmitter in accordance with an embodiment;

FIG. 8 is a third diagram of a packet/frame with pilot tones transmittedby a transmitter in accordance with an embodiment;

FIG. 9 is a diagram illustrating system performance with a modulationand coding scheme (MCS) zero (0) from the pilot dwell time table shownin FIG. 6 in accordance with one embodiment;

FIG. 10 is a diagram illustrating system performance with a MCS three(3) from the pilot dwell time table shown in FIG. 6 in accordance withone embodiment;

FIG. 11 is a second flowchart of a method for tone allocation in atransmitter in accordance with an embodiment; and

FIG. 12 is a third flowchart of a method for tone allocation in areceiver in accordance with an embodiment.

DETAILED DESCRIPTION

Various embodiments relate generally to wireless communications and moreparticularly to techniques for transmitting and receiving pilot tones.Embodiments may include improved techniques to manage pilot dwell times(N) for pilot rotation for a wireless multicarrier system. The improvedtechniques to manage pilot dwell times (N) may be advantageous for anumber of application scenarios, such as managing shifting of pilottones in a pilot tone shifting technique, managing pilot tone dwelltimes for space-time block code (STBC) techniques, managing pilot tonedwell times for transmit beamforming (TxBF) techniques, or any othercommunications techniques that may use fixed or variable pilot tonedwell times. The embodiments are not limited in this context.

An apparatus may comprise a memory configured to store a data structurewith a set of modulation and coding schemes (MCS) available to anorthogonal frequency division multiplexing (OFDM) system, such as anIEEE 802.11ah system, among others. Each MCS may have an associatedpilot dwell time (N). A pilot dwell time (N) may indicate a number ofsymbols to communicate a pilot tone on a subcarrier in a multicarriersystem before shifting the pilot tone to another subcarrier in themulticarrier system. The apparatus may further comprise a processorcircuit coupled to the memory, the processor circuit configured toidentify a MCS to communicate a packet using multiple subcarriers of theOFDM system, and retrieve a pilot dwell time (N) associated with the MCSfrom the memory. The pilot dwell time (N) may indicate when to shift apilot tone between subcarriers of the multiple subcarriers duringcommunication of the packet. In this manner, a variable pilot dwell time(N) may be used to optimize performance of an OFDM system without addingany signaling overhead, thereby conserving bandwidth, power, and othervaluable system resources. Other embodiments are described and claimed.

In a communications system, there is a need for an approach where aplatform may facilitate updating an equalizer. A transmitter transmitsone or more pilot tones in each orthogonal frequency divisionmultiplexing (OFDM) symbol set and there are typically many OFDM symbolsin a protocol data unit (PDU) or packet. With fixed pilot allocation thereceiver is able to track the received signal sufficiently accurate withthe pilot tones under most static channel conditions. According toembodiments the pilot tones may be rotated through each of thesubcarriers over the packet. The pilot tones could for example beseparated by a number of data subcarriers so as to simplify theestimation of slope and intercept for subcarrier tracking. As the pilottones are swept across the band, the taps for the equalizer for thesubcarriers for which the pilot tones currently populate would beupdated as well. This approach allows the system to track channelchanges over time when the channel is nonstationary.

According to one embodiment, a method comprises wirelessly transmittinga packet using a plurality of subcarriers; and sequentially assigningone or more pilot tones to one or more of the plurality of subcarriersduring a time period of the packet so that a communication systemreceiving the packet can track channel changes over time.

According to another embodiment, an apparatus comprises a transmissionchannel to wirelessly transmit a packet using a plurality ofsubcarriers, wherein the transmission channel sequentially assigns oneor more pilot tones to one or more of the plurality of subcarriersduring a time period of the packet; and a channel estimation modulecoupled to an input module and configured to calculate channel estimatesof the transmission channel from the one or more pilot tones; whereinsequentially assigning one or more pilot tones allows a system receivingthe packet to track transmission channel changes over time.

According to yet another embodiment, the channel estimation module in anapparatus comprises equalizer taps, the equalizer taps having an inputcoupled to an adaptive algorithm process and the equalizer taps havingan equalizer coefficients output coupled to generate channel changes.

According to another embodiment, a non-transitory machine-accessiblemedium provides instructions, which when accessed, cause a machine toperform operations, the non-transitory machine-accessible mediumcomprising code to cause at least one computer to wirelessly transmit apacket using a plurality of subcarriers and to sequentially assign oneor more pilot tones to one or more of the plurality of subcarriersduring a time period of the packet; and code to cause at least onecomputer, in a channel estimation module coupled to an input module, tocalculate channel estimates of a transmission channel from the one ormore pilot tones; wherein sequentially assigning one or more pilot tonesallows a system receiving the packet to track transmission channelchanges over time.

Exemplary embodiments are described herein. It is envisioned, however,that any system that incorporates features of any apparatus, methodand/or system described herein are encompassed by the scope and spiritof the exemplary embodiments.

Additional features and advantages of the disclosure will be set forthin the description which follows, and in part will be obvious from thedescription, or may be learned by practice of the disclosure. Thefeatures and advantages of the disclosure may be realized and obtainedby means of the instruments and combinations particularly pointed out inthe appended claims. These and other features of the present disclosurewill become more fully apparent from the following description andappended claims, or may be learned by the practice of the disclosure asset forth herein.

An algorithm, technique or process is here, and generally, considered tobe a self-consistent sequence of acts or operations leading to a desiredresult. These include physical manipulations of physical quantities.Usually, though not necessarily, these quantities take the form ofelectrical or magnetic signals capable of being stored, transferred,combined, compared, and otherwise manipulated. It has proven convenientat times, principally for reasons of common usage, to refer to thesesignals as bits, values, elements, symbols, characters, terms, numbersor the like. It should be understood, however, that all of these andsimilar terms are to be associated with the appropriate physicalquantities and are merely convenient labels applied to these quantities.

References to “one embodiment,” “an embodiment,” “example embodiment,”“various embodiments,” etc., indicate that the embodiment(s) of theinvention so described may include a particular feature, structure, orcharacteristic, but not every embodiment necessarily includes theparticular feature, structure, or characteristic. Further, repeated useof the phrase “in one embodiment” does not necessarily refer to the sameembodiment, although it may.

As used herein, unless otherwise specified the use of the ordinaladjectives “first,” “second,” “third,” etc., to describe a commonobject, merely indicate that different instances of like objects arebeing referred to, and are not intended to imply that the objects sodescribed must be in a given sequence, either temporally, spatially, inranking, or in any other manner.

Although embodiments of the invention are not limited in this regard,discussions utilizing terms such as, for example, “processing,”“computing,” “calculating,” “determining,” “applying,” “receiving,”“establishing”, “analyzing”, “checking”, or the like, may refer tooperation(s) and/or process(es) of a computer, a computing platform, acomputing system, or other electronic computing device, that manipulateand/or transform data represented as physical (e.g., electronic)quantities within the computer's registers and/or memories into otherdata similarly represented as physical quantities within the computer'sregisters and/or memories or other information storage medium that maystore instructions to perform operations and/or processes.

Although embodiments of the invention are not limited in this regard,the terms “plurality” and “a plurality” as used herein may include, forexample, “multiple” or “two or more”. The terms “plurality” or “aplurality” may be used throughout the specification to describe two ormore components, devices, elements, units, parameters, or the like. Forexample, “a plurality of resistors” may include two or more resistors.

The term “controller” is used herein generally to describe variousapparatus relating to the operation of one or more device that directsor regulates a process or machine. A controller can be implemented innumerous ways (e.g., such as with dedicated hardware) to perform variousfunctions discussed herein. A “processor” is one example of a controllerwhich employs one or more microprocessors (or processor circuits) thatmay be programmed using software (e.g., microcode) to perform variousfunctions discussed herein. A controller may be implemented with orwithout employing a processor, and also may be implemented as acombination of dedicated hardware to perform some functions and aprocessor (e.g., one or more programmed microprocessors and associatedcircuitry) to perform other functions.

The term “wireless device” as used herein includes, for example, adevice capable of wireless communication, a communication device capableof wireless communication, a mobile terminal, a communication stationcapable of wireless communication, a portable or non-portable devicecapable of wireless communication, mobile terminal, or the like.

As used herein, the term “network” is used in its broadest sense to meanany system capable of passing communications from one entity to another.Thus, for example, a network can be, but is not limited to, a wide areanetwork, a WiFi network, a cellular network, and/or any combinationthereof.

As used herein, a “sensor network” is a wireless or wired network ofnodes in which at least some of the nodes collect sensory data. Awireless sensor network (WSN) is a wireless network consisting ofspatially distributed sensors to cooperatively monitor physical orenvironmental conditions. In many situations, a plurality, majority oreven all of the nodes in a sensor network collect sensory data. Sensorydata may include external sensory data obtained by measuring/detectingnatural sources such as temperature, sound, wind, seismic activity orthe like. Sensory data may also include external sensory data obtainedby measuring/detecting man-made sources such as light, sound, variousfrequency spectrum signals, and the like. Sensory data may alternativelyinclude data related to measuring/detecting sources internal to a sensornode (e.g., traffic flow on a network, and the like).

In IEEE 802.11ah (11ah), which is a new technology evolution for WiFiand is in the standards development phase, very low data rate operationis being enabled. In IEEE 802.11a/g, 20 MHz channel widths were definedand in IEEE 802.11n 40 MHz was added and then in IEEE 802.11ac both 80and 160 MHz. In the past the evolution of WiFi has been to increase datarate, but IEEE 802.11ah actually targets comparatively lower rateservices. In IEEE 802.11ah the bandwidths defined are 1 MHz and a set ofdown-clocked IEEE 802.11ac rates, namely 2, 4, 8 and 16 MHz, where thedown clocking is 10. The 1 MHz rate is not derived from the IEEE802.11n/ac rates, and thus this bandwidth mode is being designed more orless independently. Thus far in the process, the 1 MHz system is likelyto use a 32 point FFT (as opposed to the minimum of 64 in IEEE802.11ac). Of those 32 subcarriers, it is likely that 24 will be usedfor data and 2 for pilot. Additionally, a repetition mode is beingincluded, which further lowers the data rate. It should be emphasizedthat these tone counts could change if performance requirementsnecessitate.

The identified target applications for IEEE 802.11 ah are indoor andoutdoor sensors (sensor network) and cellular offloading. It is likelythe main application will be sensor networks, e.g. smart metering. Themeasure information at each node should be delivered to a fusion centerlike an access point. In any case, in most instances the payload isanticipated to be small (hundreds of bytes), but there are several usecases that have rather large payloads (a few thousand bytes). In theselater cases, due to the low data rates of the IEEE 802.11ah system, apacket can exceed 100 milliseconds. In comparison, for the IEEE802.11n/ac system a packet length of 2400 bytes transmitted at thelowest rate takes 3.2 ms, using the highest MCS this reduces to 0.3 msand this is for only 1 stream. For these durations and the fact that thesystem was largely designed for indoor use, the channel is assumedstationary over the packet duration. With IEEE 802.11ah, which has amuch lower data rate, and has use cases targeting outdoor, thisassumption of channel stationarity is no longer valid.

The packet structure in previous versions of WiFi all have a preamble offixed duration and a few pilot tones at fixed locations. The number ofpilot tones and their location is different for the four (4) differentbandwidths of IEEE 802.11ac, but for each of the bandwidths they arefixed. The issue with potentially long packets in IEEE 802.11ah is thatin outdoor channels, the channel is not stationary over the packet. Thusadditional equalizer training or pilot phase tracking using differentpilot locations has been deemed desirable.

The approach to solve the problem was to arrive at a solution that wouldminimize the changes to the transmitter (Tx) and receiver (Rx)architecture from that of the previous IEEE 802.11a/g/n/ac versions. Thesolution outlined in this description is to use the pilot tones tocontinually update the equalizer, in addition to other receiverfunctionality. As noted above, in current versions of the standard thepackets are relatively short in time. So the use of a preamble wasjustified and, assuming a stationary channel, was efficient from anoverhead perspective. Also, with IEEE 802.11ah, where relatively lowdata rates are possible (using the lowest MCS's and single streamstransmissions), which make the packet longer in time, and where outdoorusage models are contemplated, this channel stationarity assumption isno longer valid.

In previous versions of the standard, the preamble is typically used toestimate initial receiver parameters such as frequency offsetestimation, timing estimation and such, in addition to computing theequalizer taps. The pilot tones were then typically used for trackingthrough the packet to maintain and improve frequency, time and phaseestimation. To do that, the pilot tones are currently assigned to OFDMsubcarriers in a fixed manner and then from there techniques are used toestimate these parameters across the band as needed. An example of apossible configuration for IEEE 802.11ah with fixed pilot tones at tonelocations (+7,−7) is shown in FIG. 1C.

In addition, various types of communication systems may employ one ormore of various types of signaling (e.g., orthogonal frequency divisionmultiplexing (OFDM), code division multiple access (CDMA), synchronouscode division multiple access (S-CDMA), time division multiple access(TDMA), and the like) to allow more than one user access to thecommunication system. In accordance with processing signals transmittedacross a communication channel within such communication systems, onefunction that is often performed is that of channel estimation. Fromcertain perspectives, channel estimation (variant definitions such aschannel detection, channel response characterization, channel frequencyresponse characterization, and the like) is an instrument by which atleast some characteristics of the communication channel (e.g.,attenuation, filtering properties, noise injection, and the like) can bemodeled and compensated for by a receiving communication system.

Various embodiments of the disclosure are discussed in detail below.While specific implementations are discussed, it should be understoodthat this is done for illustration purposes only. A person skilled inthe relevant art will recognize that other components and configurationsmay be used without parting from the spirit and scope of the disclosure.

The sensor network and the multi-band capable station illustrated inFIG. 1A and the related discussion are intended to provide a brief,general description of a suitable computing environment in which theinvention may be implemented. Although only three stations (STAs) areshown for simplicity, the invention is not limited to any particularnumber of STAs.

FIG. 1A illustrates a sensor network 10 in accordance to an embodiment.A wireless sensor network can be defined as a network of devices,denoted as nodes, which are capable of sensing the environment andcommunicating the information gathered from the monitored field, e.g.,an area or volume, through wireless links. The data is forwarded,possibly relays, to a controller or monitor (sink) that can use itlocally or is connected to other networks, like the Internet, through agateway. The nodes can be stationary or moving. They can be aware oftheir location or not. They can be homogeneous or not. A preferredembodiment of the present invention provides a sensor network,illustrated in FIG. 1A, as a flexible open architecture that serves as acommunication platform for multiple deployment scenarios and sensortypes. Sensors may track, for example, one or more intrusion,unauthorized, medical, or meter events. For example, a chemical sensormay take an air sample and measure its properties or a temperaturesensor can measure temperature of buildings, cars, people, objects, andthe like. A network according to a preferred embodiment, can be deployedto cover an area, indoor or outdoor, or deployed locally in rapidresponse emergency situations. Sensors can be placed in various fixed ormobile locations. Typical fixed locations include buildings,poles/towers for power or telephone lines or cellular towers or trafficlights. Typical mobile locations include vehicle such as auto,individuals, animals such as pets, or movable fixed locations.

The illustrated sensor network 10 comprises a device managementfacility/computer 160, a plurality of access points (AP) such as AP 136,also labeled 11ah AP to show that it is 11ah compliant, and a pluralityof sensor nodes, devices or stations (STAs) such as sensor node 40 in acustomer premise to perform smart metering functions, sensor node 50 tomonitor vehicle functions, sensor node 106, and sensor node 133. Awireless data collection network 170 node is shown within the network(wireless sensor network 10) to provide reachback links to existingpublic or private infrastructure types such as cellular, land mobileradio, and wired or wireless access points. A wireless data collectionnetwork 170 works as both a sensor network data concentrator as well asa reachback vehicle with existing communication infrastructures likeland mobile radio, cellular, broadband data, and the like. In essence,it provides transparent communications across different physical layers.The plurality of sensor nodes are positioned over a sensing region, andmay be individually identified as sensor nodes STA₁, STA₂, . . .STA_(N). Any particular node N of the plurality of sensor nodes is ableto communicate with one or more other sensor nodes, so as to form relaypaths to one or more of the AP nodes such as AP 136. The sensor network10 includes one or more communication devices 112 configured toestablish a wireless and/or wired communication link across wirelessdata collection network 170 with one or more remote application servers.The communication devices 112 may include a desktop, a laptop, and/or amobile computing device. Examples of mobile computing devices include,but are not limited to, a Smartphone, a tablet computer, andultra-mobile personal computers.

Device management facility/computer 160 may be located within one of theAP nodes such as AP 136, or on a server, a laptop computer, a personaldigital assistant (PDA), Smartphone, or a desktop computer. Functionsperformed by device management computer may in actual practice belocated on one computer, or distributed across several computers withdifferent programs to perform assigned individual functions. AP nodessuch as AP 136 are typical of that known in the art. AP nodes serve asthe gateway between some or all of the sensor nodes and the rest of theworld, e.g., via the Internet. An 11ah compliant AP is capable ofexchanging information with indoor and outdoor sensors and cellularoffloading. In any case, in most instances the payload is anticipated tobe small (hundreds of bytes), but there are several use cases that haverather large payloads (a few thousand bytes). In these later cases, dueto the low data rates of the 11ah system, a packet can exceed 100milliseconds. With 11ah, which has a much lower data rate, and has usecases targeting outdoor, this assumption of channel stationarity is nolonger valid and thus additional equalizer training or pilot phasetracking using different pilot locations has been deemed necessary inorder to estimate the signal across the entire data carrying portion ofthe band.

FIG. 1B is an exemplary communication device 112 suitable forimplementing different embodiments of the disclosure. The communicationdevice 112 includes a processor 186 that is coupled to one or morememory devices, such as a read only memory (ROM) 190, a random accessmemory (RAM) 188, a transceiver 182 that is coupled to a first antenna180 and to a second antenna 184, and an input/output (I/O) device 192.The processor 186 may be implemented as one or more processor chips.

Processor 186 may include, for example, a Central Processing Unit (CPU),a Digital Signal Processor (DSP), a microprocessor, a controller, achip, a microchip, an Integrated Circuit (IC), or any other suitablemulti-purpose or specific processor or controller. Processor 186 may,for example, process data received by communication device 112, and/orprocess data intended for transmission.

The ROM 190 is used to store instructions and perhaps data which areread during program execution. ROM 190 is a non-volatile memory device.The RAM 188 is used to store volatile data and perhaps to storeinstructions. The ROM 190 may include flash memories or electricallyerasable programmable memory to support updating the stored instructionsremotely, for example through an over-the-air interface via thetransceivers 182 and/or 185 and the antennas 180 and/or 184.

The transceivers 182, 185 and the antennas 180, 184 support radiocommunications. Transceivers 180 and 184 are able to perform separate orintegrated functions of receiving and/or transmitting/receiving wirelesscommunication signals, tones, blocks, frames, transmission streams,packets, messages and/or data.

The I/O device 192 may be a keypad and a visual display to permitentering numbers and selecting functions. Alternatively, the I/O device192 maybe a keyboard and a touch pad, such as a keyboard and a touch padof a laptop computer. The processor 186 executes instructions, codes,computer-executable instructions, computer programs, and/or scriptswhich it accesses from ROM 190 or RAM 188.

FIG. 1C is a diagram with fixed pilots in accordance with oneembodiment. More particularly, FIG. 1C illustrates an example of apossible configuration for IEEE 802.11ah with fixed pilot tones 194 attone locations (+7,−7).

FIG. 2 is a diagram of a packet generated as a function of time withpilot tones transmitted by a transmitter in accordance with anembodiment. FIG. 2 shows a signal that comprises an OFDM symbol set 202.Each OFDM symbol set includes multiple data symbols modulated bydistinct subcarriers 204 (e.g., subcarrier frequencies). Each OFDMsymbol set includes pilot tones 210, data symbols 205, guard subcarriers211 and 213, DC subcarriers (0 Hz) 212 although other configurations arepossible. The DC and guard subcarriers are sometimes collectively calledthe null subcarriers/tones (null tones). Null tones are used in OFDMsystems to protect against DC offset (DC subcarrier) and to protectagainst adjacent channel interference (guard subcarriers). Additionally,guard subcarriers are left blank to allow for fitting the transmittedwaveform into a transmit spectral mask with less costly implementation.

The pilot tones according to an embodiment may be assigned to one ormore usable carriers (i.e. carriers not including guard or DC tones)such that, as shown by way of FIG. 2, they sweep through the usablecarriers as a function of time, such as through all usable subcarriers.The pilot tones 210 may be modulated by the same sub-subcarrierfrequency in each of the OFDM symbol sets but disposed at differentsub-subcarrier positions in different symbol sets. In a sequentialassignment of pilot tones, difference in position (P), spacing 215,between the pilot tones in the same symbol sets may be such that every n(n>=1) symbol position in a symbol set is occupied by a pilot tone. Asshown the spacing between the pilot tones is fourteen (14) subcarriersand this fixed position may be maintained for each symbol set. FIG. 3illustrates an alternative strategy where the spacing varies as a resultof random assignment employed on the positioning of the pilot tones.

The pilot tones are disposed at different sub-subcarrier positions indifferent symbol sets through time by way of pilot tone shifting. Pilottone shifting is a process where the pilot tones may be sequentially orrandomly assigned to different sub-subcarrier as a function of time. Aspreviously mentioned, only a subset of subcarriers may be used for pilotor usable carriers. For example, the pilot tones may be used only ondata subcarriers (e.g., sweep through with the pilot tone on a symbol bysymbol basis), avoid nulled subcarriers (e.g., DC subcarriers and guardsubcarriers), and potentially even avoid data tones that are adjacent toguard or DC subcarriers. The pilot tones and their positioning can bebased on channel conditions such as coding scheme, packet length, andthe like. As shown on time axis 290, PT₁ (time=1 or a first time periodof the packet being generated) the position of the pilot tones are −13and 1; while at PT₂ (time=2) the positions are shifted by one and thepilot tones are assigned to −12 and 2. As shown the pilot tones 205 areshifted 220 one position in the time domain. The pilot tones could beshifted such that there is a shift every symbol set as shown, or couldstay fixed for several symbol sets and then be shifted. The shifting ofthe pilot tones 210 can be based on the modulation and coding scheme(MCS) used for transmission or on the packet length of the transmission(i.e., channel conditions). Further, the amount of time the one or morepilot tones 210 occupy at a particular subcarrier could be based on amodulation and coding scheme (MCS), the MCS selected based on a datarate and a level of robustness required by traffic type. After a set ofpilot tones are assigned, the process 292 of assigning pilot tones isrepeated for each time period of a plurality of time periods.

FIG. 3 illustrates a method 300 for random pilot shifting as function oftime in an OFDM-based communication system in accordance to anembodiment. This diagram shows multiple frames, at different times, ofan OFDM signal with each frame including pilot tones 210, data tones 410and 420, and null tones which are generally found at (−16, −15, 0, 14,and 15) for the 1 MHz bandwidth case example. While in a wirelessnetwork sensor a uniform modulation is used for all the data tones, anOFDM signal may comprise data tones 402 of different modulation types.Example of different modulation types are Quadrature phase-shift keying(QPSK) and Binary phase-shift keying (BPSK) which is of a relativelylower modulation order than QPSK. In FIG. 3, tone set (tones −12 and−11) may use a QPSK modulation type and there may be an even greaterconfidence associated with a symbol extracted from that data tone toqualify it as a pseudo-pilot tone. Tone set (tones 10 and 11) could bedata tones whose corresponding symbols have relatively lower modulationorder types (such as below, e.g., 16 QAM, BPSK, and the like) mayqualify more frequently for pilot tone insertion than data tones whosecorresponding symbols have relatively higher modulation order types likeQPSK.

Additionally, the amount of time the pilot tones occupy a particularsubcarrier could be dependent on modulation and coding scheme (MCS). Forexample in 0.11ah, where a new BPSK rate 1/2 mode is defined with arepetition coding of 2×, the fixed duration could be longer than that ofthe MCS0, BPSK rate 1/2 mode which has no repetition.

Finally, the approach allows the system to use fixed pilot tones forpackets which are short in duration as in previous versions of thestandard so as to minimize the processing. Thus, it allows the option ofusing the technique in all packet transmissions, or to only be used forconfigurations such as low MCS's with 1-stream and large payloads. UsingMCS and packet length to determine the setting for the pilot rotationallows a simple design since these parameters are signaled in thepreamble in the signal field(s).

FIG. 4 illustrates part of a transceiver 182 with equalizer forprocessing pilot tones and data tones in accordance to an embodiment.Receiver 182 comprises an antenna 180, an input module 412, an adaptiveequalizer 220 running an equalizer application 240 or instructions, andchannel estimation module 230.

Input module 412 includes an interface to provided signals to adaptiveequalizer 440 and other circuits from antenna 180. Input module maycomprise filters, delay elements, and taps with their correspondingcoefficients to provide an output which depends on the instantaneousstate of the radio channel.

The tap coefficients are weight values which may be adjusted based onthe pilot tones to achieve a specific level of performance, andpreferably to optimize signal quality at the receiver. In oneembodiment, the receiving system is able to track channel changes overtime (e.g., using the pilot tones to update the equalizer taps) becauseof the rotation of the pilot tones through each of the OFDM subcarriersover the packet through time. As noted above, the pilot tones areseparated by some number of data subcarriers so that estimation of slopeand intercept for subcarrier is simplified. As the pilot tones are sweptacross the band, the taps for the equalizer for the subcarriers forwhich the pilot tones currently populate may be updated as well.

The pilot tones 210 are received at antenna 180 and converted to abaseband representation by input module 412. The received pilot tonesare then input into the channel estimator 436 which uses the receivedsequences to determine initial channel estimates for the wirelesschannel (using, for example, a least squares approach). The channelestimator 436 may have a priori knowledge of the transmitted pilot toneswhich it compares to the received signals to determine the initialchannel estimates. The initial channel estimates may then be deliveredto the channel tracking unit 438. The data signals are received by theantenna 180 and converted to a baseband representation within thetransceiver 182 input module 412. The data signals are then delivered tothe input of the equalizer 440 which filters the signals in a mannerdictated by the channel taps currently being applied to the equalizer440. The equalizer 440 may include any type of equalizer structure(including, for example, a transversal filter, a maximum likelihoodsequence estimator (MLSE), and others). When properly configured, theequalizer 440 may reduce or eliminate undesirable channel effects withinthe received signals (e.g., inter-symbol interference).

The received data signals with pilot tones 210 are also delivered to theinput of the channel tracking unit 438 which uses the received signalsto track the channel taps applied to the equalizer 440. During systemoperation, these taps are regularly updated by the channel tracking unit438 based on the magnitude and phase of the pilot tones. In addition tothe receive data, the channel tracking unit 438 also receives data froman output of the equalizer 440 as feedback for use in the channeltracking process. The channel tracking unit 438 uses the initial channelestimates determined by the channel estimator 436 to determine thechannel taps covariance matrix (C). In one embodiment, for example,channel tracking unit 438 then determines the value of the constant b(related to the channel changing rate) and calculates the taps changingcovariance matrix (b*C). The square root of the taps changing covariancematrix is then determined and used within a modified least mean square(LMS) algorithm to determine the updated channel taps, which are thenapplied to the equalizer 440. The output of the equalizer 440 isde-interleaved in the de-interleaver 442. Channel and source coding isthen removed from the signal in the channel decoder 444 and the sourcedecoder 446, respectively. The resulting information is then deliveredto the information sink 448 which may include a user device, a memory,or other data destination as shown by output 250.

FIG. 5 is a flowchart of a method for tone allocation in a transmitterin accordance to an embodiment. Method 500 begins with action 510 and isrepeated for every packet. In action 510, a device such as communicationdevice 112 wirelessly transmits a packet using a plurality ofsubcarriers that may include pilot, data, and null tones. Control isthen passed to action 520 where the process assigns one or more pilottones to the plurality of subcarriers. The assignment of the one or morepilot tones in action 520 is done in conjunction with action 530 thatshifts the one or more pilot tones a number of subcarriers from aprevious position on the packet. Control is then returned to action 520where the pilot tones are assigned to particular subcarriers of the OFDMsignal. Control is then passed to action 510 where wirelesscommunication is conducted by the communication device. The shifting ofthe pilot tones as noted earlier could be either fixed, for example ashift every symbol, variably shifted where the pilot tones stay fixedfor several symbols and then varied, or it could be randomly shifted inaccordance to a uniform distribution.

FIG. 6 illustrates an exemplary pilot dwell time table 600 in accordancewith one embodiment. The pilot dwell time table 600 may store, amongother types of information, a set of MCS available to an OFDM system,each MCS having an associated pilot dwell time (N). A pilot dwell time(N) may indicate a number of symbols to communicate a pilot tone 210 ona subcarrier 204 in a multicarrier system before shifting the pilot tone210 to another subcarrier 204 in the multicarrier system. The pilotdwell time table 600 may be stored as any type of data structure in astorage medium, such as RAM 188, ROM 190, and other storage mediumssuitable for use with an OFDM system and OFDM devices. Although referredto as a pilot dwell time table 600, it may be appreciated that theinformation described for the pilot dwell time table 600 may be storedin any data structure, such as an array, linked list, database,relational database, lookup table (LUT), and so forth. The embodimentsare not limited in this context.

It may be appreciated that although some embodiments describe the use ofpilot dwell time (N) and the pilot dwell time table 600 in the contextof managing shifting of pilot tones for one or more pilot tone shiftingtechniques, the pilot dwell time (N) and the pilot dwell time table 600may be used for other applications, such as for managing pilot tonedwell times for space-time block code (STBC) techniques, managing pilottone dwell times for transmit beamforming (TxBF) techniques, or anyother communications techniques that may use fixed or variable pilottone dwell times. For example, there are other transmit modes thatresult in different operating conditions (e.g., SNR), and thus would usedifferent N values due to the varying operating conditions. With TxBF,for example, the values given with the pilot dwell time table 600 asshown in FIG. 6 could be used, with N incremented or decremented by oneor more integers. With STBC, for example, different N values could beused for different STBC modes. Additionally, the use of differentencoders could result with different N values. For instance,convolutional encoders may use the values given with the pilot dwelltime table 600 as shown in FIG. 6, while LDPC encoders may use thevalues given with the pilot dwell time table 600 as shown in FIG. 6 anddecremented by one or more. The embodiments are not limited to theseexamples.

As previously described with reference to FIGS. 1-5, pilot tones 210 maybe disposed at different sub-subcarrier positions in different OFDMsymbol sets 202 through time by way of pilot tone shifting. Pilot toneshifting is a process where the pilot tones 210 may be sequentially orrandomly assigned to different subcarrier 204 as a function of time. Thepilot tones could be shifted such that there is a shift every symbol setas shown in FIG. 2, or could stay fixed for several symbol sets and thenbe shifted. In the latter case, the amount of time pilot tones 210occupy a particular subcarrier 204 may be indicated by a pilot dwelltime (N) stored in the pilot dwell time table 600.

With pilot tone shifting (or pilot tone rotation), a pilot tone 210 isshifted to a new location every N symbols, where N is a systemparameter. Thus, the pilot tone 210 remains constant for N symbols, thenshifts to the next location. A receiver may then use the N pilot symbolsto make a channel estimate using an appropriate algorithm. The systemcould be designed with a single fixed value of N, but that does notallow for optimization.

In various embodiments, the sensor network 10 may use several values forN, where N is any positive integer. In one embodiment, for example,values for N may range from 1 to 8 OFDM symbols. Using different valuesfor N may allow a pilot tone 210 to be communicated on a particularsubcarrier 204 for varying amounts of time. A larger value for N mayindicate a greater amount of time a pilot tone 210 is communicated on asubcarrier 204, which provides a longer integration time and potentiallyhigher signal-to-noise ratio (SNR) for an estimate. Conversely, asmaller value for N may indicate a lesser amount of time a pilot tone210 is communicated on a subcarrier 204, which provides a shorterintegration time and potentially lower SNR for an estimate. Therefore, Nmay be customized for a particular packet, media, channel, device, orsystem to improve overall performance.

One problem associated with using a variable N, however, is that areceiver needs to be informed about the value of N (e.g., the dwell timebefore a pilot rotation or shift) that will be used in a packet. Oneapproach is to signal this information to the physical (PHY) layer usinga signal (SIG) field of a preamble. A major drawback of this approach isthat signaling of 1 to 8 values would require 3 bits in the SIG field.Unfortunately for a 1 MHz system, there are very few data tones and withrepetition, adding an extra symbol equates to adding 2 symbols withrepetition. Even if a 1 MHz system would have additional bits to signala value for N, this would increase signaling traffic in a networkthereby consuming more bandwidth and other network resources.

Various embodiments provide a technique for a multicarrier system toutilize a variable pilot dwell time that is automatically known to botha transmitter and receiver through other system parameters, whilereducing or eliminating the need to signal the variable pilot dwell timeto either the transmitter or the receiver. In one embodiment, forexample, this may be accomplished by associating fixed pilot dwell times(N) with a MCS used for a packet, as shown by the pilot dwell time table600 of FIG. 6. The design trade-off for pilot tone shifting systems (andother systems such as STBC, TxBF or channel coding types) is that forstationary channels, a larger N indicates a longer dwell time andsubsequent better performance. Since the channel is stationary, thelonger integration gives a better SNR for an estimate, as demonstratedin FIGS. 9, 10, which shows that performance is better than a systemwith no pilot rotation (e.g., N>4). This is because integration time foreach pilot tone 210 is longer than an original preamble which was usedfor the initial channel estimate for all pilot tones 210. With theaddition of Doppler, longer integration times can start to degradeperformance relative to shorter integration times. As a note, even longintegration times are better than not using pilot tone rotation as in802.11n/ac systems. Nonetheless, it is useful to have N configured inorder to optimize the system, but without adding additional overheadwith signaling.

Referring again to FIG. 6, the pilot dwell time table 600 may store,among other types of information, a set of MCS available to an OFDMsystem, each MCS having an associated pilot dwell time (N). In oneembodiment, selecting a pilot dwell time (N) to associate with a givenMCS may be empirically derived based on historical information for thesensor network 10, and encoded in the pilot dwell time table 600. Valuesfor the pilot dwell time table 600 stored in memory of various devices(e.g., sensor nodes 40, 50, 106, and/or 133) may be updated on aperiodic, aperiodic, continuous, or on-demand basis.

In some cases, it may be possible so select a value of N to associatewith a given MCS based on instantaneous channel information, and updatethe values of the pilot dwell time table 600 stored in memory of variousdevices (e.g., sensor nodes 40, 50, 106, and/or 133) accordingly.However, this approach has some design trade-offs. Selecting a value forN based on instantaneous channel information is very difficult in anIEEE 802.11ah system which has a main use case of low power sensors. Forexample, these devices exchange information infrequently andadditionally are typically very low power devices, so a designconstraint is to minimize their time “awake.” Further, frequent updateswould add additional overhead to all transmissions, even those wherepilot rotation is not enabled, thereby impacting the system throughputand device power consumption.

The pilot dwell time table 600 may include, among other types ofinformation, a MCS field 602, a modulation field 604, a code rate field606, and a pilot dwell time (N) field 608. The MCS field 602 may store acode index for a particular type of MCS, such as MCS0 to MCS9, forexample. The modulation field 604 may store a modulation type associatedwith each code index, such as binary phase-shift keying (BPSK),quadrature phase-shift keying (QPSK), 16 quadrature amplitude modulation(QAM) (16-QAM), 64-QAM, 256-QAM, and so forth. The code rate field 606may store a code rate of a convolutional code associated with each codeindex, such as 1/2, 2/3, 3/4, 5/6 and so forth. The pilot dwell time (N)field 608 may store an integer value for N, such as 1-8 symbols. In thisconfiguration, a code index from the MCS field 602 may indicatedifferent types of associated information. For instance, a code index610 of MCS4 may be associated with a modulation type of 16-QAM, a 3/4code rate, and N=2. It may be appreciated that the fields and valuesshown in the pilot dwell time table 600 are merely examples, and otherfields and values may be implemented for a given pilot dwell time table600. For instance, a field (not shown) may be added to the pilot dwelltime table 600 to indicate a pilot tone shifting pattern, such assequential or random, for instance.

FIG. 7 is a diagram of a packet generated as a function of time withpilot tones transmitted by a transmitter in a sequential manner. Aspreviously described with reference to FIG. 2, pilot tones 210 may bedisposed at different sub-subcarrier positions in different symbol setsthrough time by way of pilot tone shifting. In one embodiment, the pilottones 210 could be shifted to different subcarriers as indicated by apilot dwell time (N) stored in the pilot dwell time table 600. The pilotdwell time table 600 may be stored in both a transmitting device and areceiving device. In this manner, once the transmitting device and thereceiving device select or agree on a MCS for a channel or packet, suchas through a rate adaptation process to converge on an optimal MCS froma throughput perspective, the transmitting device and the receivingdevice may retrieve a pilot dwell time (N) associated with the selectedMCS from local pilot dwell time tables 600 without any additionalsignaling exchanged between the devices.

In one embodiment, for example, a processor circuit (e.g., processor186) for a transmitting device and/or a receiving device may beconfigured to identify a MCS to communicate a packet using multiplesubcarriers 204 of an OFDM system, such as sensor network 10. Theprocessor circuit may retrieve a pilot dwell time (N) from the pilotdwell time field 608 associated with the identified MCS from the pilotdwell time table 600 stored in memory. The pilot dwell time (N) mayindicate when to shift a pilot tone 210 between subcarriers 204 duringcommunication of the packet. In one embodiment, for example, the pilotdwell time (N) may indicate a shift of a pilot tone 210 from a firstsubcarrier 204 ₁ to a second subcarrier 204 ₂ of the multiplesubcarriers 204 every 1 to 8 OFDM symbols. However, the embodiments arenot limited to these values.

Pilot tone shifting may occur in either a sequential or random manner.This may be a configurable parameter stored by the transmitting deviceand receiving device, such as through another field added to the pilotdwell time table 600. Alternatively, in addition to the pilot dwell time(N) indicating a shift of a pilot tone 210 from a first subcarrier 204 ₁to a second subcarrier 204 ₂, the pilot dwell time (N) may furtherindicate whether the shift between subcarriers 204 should occur in asequential or random manner. For instance, certain values for N mayindicate sequential shifts (e.g., when N=1 to 4), while other values forN may indicate random shifts (e.g., when N=5 to 8). Embodiments are notlimited in this context.

FIG. 7 illustrates a case of pilot shifting when N=2 and sequentialshifts. As shown on time axis 290, at PT₁ (time=1 or a first time periodof the packet) the position of the pilot tones 210 of the OFDM symbolset 202 are −13 and 1. At PT₂ (time=2), the position of the pilot tones210 remain at −13 and 1 as indicated by N=2. At PT₃ (time=3), thepositions are shifted by one and the pilot tones 210 are assigned to −12and 2. As shown the pilot tones 210 are shifted 220 one position in thetime domain. At PT₄ (time=4), the position of the pilot tones 210 remainat −12 and 2, again as indicated by N=2. After a set of pilot tones 210are assigned, the process 292 of assigning pilot tones is repeated foreach time period of a plurality of time periods in a sequential manner.

FIG. 8 is a diagram of a packet generated as a function of time withpilot tones transmitted by a transmitter in a random manner. Moreparticularly, FIG. 8 illustrates a case of pilot shifting when N=2 andrandom shifts. As shown on time axis 290, at PT₁ (time=1) the positionof the pilot tones 210 are −13 and 1. At PT₂ (time=2), the position ofthe pilot tones 210 remain at −13 and 1 as indicated by N=2. At PT₃(time=3), the positions are shifted by a random number of positions andthe pilot tones 210 are assigned to −10 and 4. As shown the pilot tones210 are shifted 220 three positions in the time domain. At PT₄ (time=4),the position of the pilot tones 210 remain at −10 and 4, again asindicated by N=2. At PT₅ (time=5), the positions are again shifted by arandom number of positions and the pilot tones 210 are assigned to −5and 9. As shown the pilot tones 210 are shifted 220 five positions inthe time domain. At PT₆ (time=6), the position of the pilot tones 210remain at −5 and 9, again as indicated by N=2. After a set of pilottones 210 are assigned, the process 292 of assigning pilot tones isrepeated for each time period of a plurality of time periods in a randommanner.

It may be appreciated that in FIGS. 7, 8, the spacing between pilottones 210 for a given OFDM symbol set 202 remain a fixed number ofpositions apart, which in this case is fourteen (14) subcarriers,regardless of whether the pilot tone shifts are sequential or random.Alternatively, in some cases, the spacing between pilot tones 210 mayvary as well. The embodiments are not limited in this context.

FIG. 9 is a diagram illustrating system performance with a MCS0 from thepilot dwell time table 600 shown in FIG. 6. A study was done todetermine the appropriate selection of the pilot rotation dwell time(N), and the MCS used. For brevity only a few cases are shown here toprovide insight to the final selection of N in the pilot dwell timetable 600. FIG. 9 illustrates system performance with MCS0 (BPSK rate1/2). As can be seen in FIG. 9, to attain sufficient integration a totalof 4 symbols (e.g., N=4) are needed to match the performance with nopilot rotation. This is a positive result since the preamble used forinitial channel estimation was also 4 symbols long and used BPSKsignaling.

FIG. 10 is a diagram illustrating system performance with a MCS3 fromthe pilot dwell time table 600 shown in FIG. 6. FIG. 10 illustratessystem performance with MCS3 (16-QAM rate 1/2). As can be seen in FIG.10, MCS3 utilizes 16-QAM and therefore requires a higher SNR to meet apacket error rate (PER) target. As such MCS3 only requires anintegration time of N=2 to match the performance with no pilot rotation(e.g., such as an 802.11n/ac system). Thus, integration time beyond thisis not justified. This allows better Doppler tracking without comprisingthe system in stationary channels. Based on the study, the pilot dwelltime table 600 was created and is proposed as inclusion in the 802.11ahstandard. The approach is to signal the receiver that pilot rotation isused, and once it is determined that pilot rotation is to be used in thetransmitter, it will use the N value based on the MCS selection asoutlined in the pilot dwell time table 600. It is worthy to note thatpilot rotation is not necessarily used in each packet, and is typicallybased on the packet time on air.

FIG. 11 is a flowchart of a method 1100 for tone allocation in atransmitter in accordance with an embodiment. For instance, the method1100 may be utilized in various transmitting devices (e.g., sensor nodes40, 50, 106, and/or 133) via the transceiver 182.

As shown in FIG. 11, method 1100 may identify a MCS for a packet of anOFDM system at block 1102. For instance, a sensor node (e.g., sensornodes 40, 50, 106, and/or 133) may identify a MCS for a packet of anOFDM system through a rate adaptation process.

The method 1100 may retrieve a pilot dwell time associated with the MCSfrom a storage medium, the pilot dwell time to represent a length oftime a pilot tone is communicated on a subcarrier of the OFDM systembefore the pilot tone is shifted to a different subcarrier of the OFDMsystem, at block 1104. For instance, the processor 186 may retrieve apilot dwell time (N) associated with the MCS from the pilot dwell timetable 600 stored in RAM 188 or ROM 190. The pilot dwell time (N) mayindicate a length of time a pilot tone 210 is communicated on asubcarrier 204 of the sensor network 10 before the pilot tone 210 isshifted to a different subcarrier 204 of the sensor network 10. In oneembodiment, the pilot dwell time (N) may comprise a number of OFDMsymbols to communicate the pilot tone 210 on each subcarrier 204, suchas 1 to 8 OFDM symbols, for example.

The method 1100 may transmit the packet using the multiple subcarriersat block 1106. For example, the transceiver 182 may transmit an OFDMsymbol set 202 using the multiple subcarriers 204.

The method 1100 may assign a pilot tone to a first subcarrier of themultiple subcarriers during transmission of the packet at block 1108.For example, the processor 186 may assign a pilot tone 210 to a firstsubcarrier 204 ₁ of the multiple subcarriers during transmission of theOFDM symbol set 202 at a first time instance.

The method 110 may shift the pilot tone from the first subcarrier to asecond subcarrier of the multiple subcarriers based on the pilot dwelltime during transmission of the packet at block 1110. For example, theprocessor 186 may cause the transceiver 182 to shift the pilot tone 210from the first subcarrier 204 ₁ to a second subcarrier 204 ₂ of themultiple subcarriers 204 based on the pilot dwell time (N) duringtransmission of the OFDM symbol set 202 at a second time instance, withan amount of time between the first and second time instances determinedby N. In one embodiment, the pilot tone shift may occur in a sequentialmanner. In one embodiment, the pilot tone shift may occur in a randommanner.

FIG. 12 is a flowchart of a method 1200 for tone allocation in areceiver in accordance with an embodiment. For instance, the method 1200may be utilized in various receiving devices (e.g., sensor nodes 40, 50,106, and/or 133) via the transceiver 182.

As shown in FIG. 12, method 1200 may identify a MCS for a packet of anOFDM system at block 1202. For instance, a sensor node (e.g., sensornodes 40, 50, 106, and/or 133) may identify a MCS for a packet of anOFDM system through a rate adaptation process.

The method 1100 may retrieve a pilot dwell time associated with the MCSfrom a storage medium, the pilot dwell time to represent a length oftime a pilot tone is communicated on a subcarrier of the OFDM systembefore the pilot tone is shifted to a different subcarrier of the OFDMsystem, at block 1204. For instance, the processor 186 may retrieve apilot dwell time (N) associated with the MCS from the pilot dwell timetable 600 stored in RAM 188 or ROM 190. The pilot dwell time (N) mayindicate a length of time a pilot tone 210 is communicated on asubcarrier 204 of the sensor network 10 before the pilot tone 210 isshifted to a different subcarrier 204 of the sensor network 10. In oneembodiment, the pilot dwell time (N) may comprise a number of OFDMsymbols to communicate the pilot tone 210 on each subcarrier 204, suchas 1 to 8 OFDM symbols, for example.

The method 1200 may receive the packet using the multiple subcarriers atblock 1206. For instance, the transceiver 182 may receive an OFDM symbolset 202 using the multiple subcarriers 204.

The method 1200 may receive pilot tones on different subcarriers of themultiple subcarriers during receipt of the packet based on the pilotdwell time at block 1208. For example, the transceiver 182 may receivepilot tones 210 on different subcarriers 204 ₁, 204 ₂ of the multiplesubcarriers 204 during receipt of the OFDM symbol set 202 at differenttime instances based on the pilot dwell time (N). Assume a transmittingdevice utilizes a known MCS and the processor 186 assigns a pilot tone210 to a first subcarrier 204 ₁ of the multiple subcarriers 204 duringtransmission of the OFDM symbol set 202 at a first time instance for atime period defined by N. For instance, when N=2, the transceiver 182will transmit the pilot tone 210 on the first subcarrier 204 ₁ for aperiod of 2 symbols. Meanwhile, the processor 186 of the receivingdevice, having knowledge of the MCS used by the transmitting device,will retrieve a value for N associated with the MCS from the pilot dwelltime table 600, and direct the transceiver 182 to monitor the firstsubcarrier 204 ₁ for the pilot tone 210 for a period of time defined byN. For instance, when N=2, the transceiver 182 will monitor the firstsubcarrier 204 ₁ to receive the pilot tone 210 for a period of 2symbols. After 2 symbols, the transmitting device may shift the pilottone 210 from the first subcarrier 204 ₁ to a second subcarrier 204 ₂ ofthe multiple subcarriers 204. The processor 186 of the receiving device,having knowledge of N derived through the known MCS used by thetransmitting device, will monitor the second subcarrier 204 ₂ to receivethe pilot tone 210 for a period of time defined by N, which in thisexample is 2 symbols. This pilot tone shifting process will continueuntil the packet is completely transmitted and received by transceiver182.

Thus, embodiments using different pilot dwell times (N) based on MCSprovides some level of optimization without adding undo overhead to theentire system. It adds this optimization without adding two (2)additional SIG field symbols and without needing the devices to exchangeinformation in multiple transmissions which would reduce battery life ofthe devices.

Embodiments within the scope of the present disclosure may also includecomputer-readable media for carrying or having computer-executableinstructions or data structures stored thereon. Such computer-readablemedia can be any available media that can be accessed by a generalpurpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium which can be used to carryor store desired program code means in the form of computer-executableinstructions or data structures. When information is transferred orprovided over a network or another communications connection (eitherhardwired, wireless, or combination thereof) to a computer, the computerproperly views the connection as a computer-readable medium. Thus, anysuch connection is properly termed a computer-readable medium.Combinations of the above should also be included within the scope ofthe computer-readable media.

Computer-executable instructions include, for example, instructions anddata which cause a general purpose computer, special purpose computer,or special purpose processing device to perform a certain function orgroup of functions. Computer-executable instructions also includeprogram modules that are executed by computers in stand-alone or networkenvironments. Generally, program modules include routines, programs,objects, components, and data structures, etc. that performs particulartasks or implement particular abstract data types. Computer-executableinstructions, associated data structures, and program modules representexamples of the program code means for executing steps of the methodsdisclosed herein. The particular sequence of such executableinstructions or associated data structures represents examples ofcorresponding acts for implementing the functions described in suchsteps.

Various processes to support the establishment of channel estimation andtracking. Using the disclosed approach, efficient and productive use ofcomputing resources in a communication device to track channel changesover time by assigning one or more pilot tones to a packet. Although theabove description may contain specific details, they should not beconstrued as limiting the claims in any way. Other configurations of thedescribed embodiments of the disclosure are part of the scope of thisdisclosure. For example, the principles of the disclosure may be appliedto each individual user where each user may individually deploy such asystem. This enables each user to utilize the benefits of the disclosureeven if any one of the large number of possible applications do not needthe functionality described herein. In other words, there may bemultiple instances of the components each processing the content invarious possible ways. It does not necessarily need to be one systemused by all end users. Accordingly, the appended claims and their legalequivalents should only define the disclosure, rather than any specificexamples given.

1. An apparatus configured to communicate orthogonal frequency divisionmultiplexing (OFDM) wireless communications, the apparatus comprising: aprocessor circuit configured to vary, in a non-sequential manner,positions of first and second pilot tones between a plurality of symbolsof a packet, a symbol of the plurality of symbols comprising a first setof subcarriers and a second set of subcarriers after the first set ofsubcarriers, said processor circuit is configured to set the first pilottone to subcarriers within the first set of subcarriers and to set thesecond pilot tone to subcarriers within the second set of subcarriers,while maintaining a predefined constant spacing between the first andsecond pilot tones; and a transceiver to transmit an electromagneticrepresentation of the packet.
 2. The apparatus of claim 1, wherein thefirst set of subcarriers comprises thirteen subcarriers denoted (−1) to(−13), and the second set of subcarriers comprises thirteen subcarriersdenoted (1) to (13).
 3. The apparatus of claim 2, wherein said processorcircuit is configured to set the first and second pilot tones to aplurality of pairs of subcarriers comprising at least a pair ofsubcarriers (−10,4), a pair of subcarriers (−5, 9), and a pair ofsubcarriers (−13,1).
 4. The apparatus of claim 1, wherein the spacingbetween the first and second pilot tones comprises a first number of thefirst set of subcarriers and a second number of the second set ofsubcarriers, and wherein a sum of the first and second numbers isconstant between said symbols.
 5. The apparatus of claim 1, wherein theconstant spacing comprises a spacing of thirteen subcarriers.
 6. Theapparatus of claim 1, wherein the symbol comprises the first and secondpilot tones, a plurality of data subcarriers, a plurality of guardsubcarriers and a Direct Current (DC) subcarrier.
 7. The apparatus ofclaim 1, wherein the transceiver is configured to operate according tospace time block code (STBC) techniques.
 8. The apparatus of claim 1,wherein the transceiver is configured to communicate in a low ratecommunication and sensors networks.
 9. The apparatus of claim 1comprising a sensor node.
 10. The apparatus of claim 1 comprising one ormore antennas, and a memory.
 11. An apparatus configured to communicateorthogonal frequency division multiplexing (OFDM) wirelesscommunications, the apparatus comprising: a transceiver to receive apacket comprising a plurality of symbols, a symbol of the plurality ofsymbols comprising a first set of subcarriers and a second set ofsubcarriers after the first set of subcarriers, the symbols comprising afirst pilot tone at subcarriers within the first set of subcarriers anda second pilot tone at subcarriers within the second set of subcarriers,wherein positions of the first and second pilot tones vary, in anon-sequential manner, between the plurality of symbols of the packet,while maintaining a predefined constant spacing between the first andsecond pilot tones; and a processor circuit configured to trigger thetransceiver to monitor the plurality of symbols of the packet for thefirst and second pilot tones.
 12. The apparatus of claim 11, wherein thefirst set of subcarriers comprises thirteen subcarriers denoted (−1) to(−13), and the second set of subcarriers comprises thirteen subcarriersdenoted (1) to (13).
 13. The apparatus of claim 12, wherein the firstand second pilot tones are at a plurality of pairs of subcarrierscomprising at least a pair of subcarriers (−10,4), a pair of subcarriers(−5, 9), and a pair of subcarriers (−13,1).
 14. The apparatus of claim11, wherein the spacing between the first and second pilot tonescomprises a first number of the first set of subcarriers and a secondnumber of the second set of subcarriers, and wherein a sum of the firstand second numbers is constant between said symbols.
 15. The apparatusof claim 11, wherein the constant spacing comprises a spacing ofthirteen subcarriers.
 16. The apparatus of claim 11, wherein the symbolcomprises the first and second pilot tones, a plurality of datasubcarriers, a plurality of guard subcarriers and a Direct Current (DC)subcarrier.
 17. The apparatus of claim 11, wherein the transceiver isconfigured to operate according to space time block code (STBC)techniques.
 18. The apparatus of claim 11 comprising one or moreantennas, and a memory.
 19. A product comprising a non-transitorycomputer readable storage medium having a computer readable program codeembodied therein, the computer readable program code configured to beexecuted by a communication device to perform one or more operations,the operations comprising: varying, in a non-sequential manner,positions of first and second pilot tones between a plurality of symbolsof a packet, a symbol of the plurality of symbols comprising a first setof subcarriers and a second set of subcarriers after the first set ofsubcarriers, said varying comprises setting the first pilot tone tosubcarriers within the first set of subcarriers and setting the secondpilot tone to subcarriers within the second set of subcarriers, whilemaintaining a predefined constant spacing between the first and secondpilot tones; and transmitting an electromagnetic representation of thepacket.
 20. The product of claim 19, wherein the first set ofsubcarriers comprises thirteen subcarriers denoted (−1) to (−13), andthe second set of subcarriers comprises thirteen subcarriers denoted (1)to (13).
 21. The product of claim 20, wherein the operations comprisesetting the first and second pilot tones to a plurality of pairs ofsubcarriers comprising at least a pair of subcarriers (−10,4), a pair ofsubcarriers (−5, 9), and a pair of subcarriers (−13,1).
 22. The productof claim 19, wherein the spacing between the first and second pilottones comprises a first number of the first set of subcarriers and asecond number of the second set of subcarriers, and wherein a sum of thefirst and second numbers is constant between said symbols.
 23. A productcomprising a non-transitory computer readable storage medium having acomputer readable program code embodied therein, the computer readableprogram code configured to be executed by a communication device toperform one or more operations, the operations comprising: receiving apacket comprising a plurality of symbols, a symbol of the plurality ofsymbols comprising a first set of subcarriers and a second set ofsubcarriers after the first set of subcarriers, the symbols comprising afirst pilot tone at subcarriers within the first set of subcarriers anda second pilot tone at subcarriers within the second set of subcarriers,wherein positions of the first and second pilot tones vary, in anon-sequential manner, between the plurality of symbols of the packet,while maintaining a predefined constant spacing between the first andsecond pilot tones; and monitoring the plurality of symbols of thepacket for the first and second pilot tones.
 24. The product of claim23, wherein the first set of subcarriers comprises thirteen subcarriersdenoted (−1) to (−13), and the second set of subcarriers comprisesthirteen subcarriers denoted (1) to (13).
 25. The product of claim 24,wherein the first and second pilot tones are at a plurality of pairs ofsubcarriers comprising at least a pair of subcarriers (−10,4), a pair ofsubcarriers (−5, 9), and a pair of subcarriers (−13,1).